With the release of the 1.5 Ships update, Elite has had an additional five ships added to the game and the previous scale diagram I'd done was was both outdated and missing several keys ships - so especially for you, I have re-drawn the whole thing using all new graphics and the most up-to-date size information.
The diagram uses a scale of 1 pixel to 1 metre.
Credit goes to Mat Ricardo (Prefim on Reddit) for creating the ship renders.
UPDATE: Now with more capital ship goodness! Thanks to ElitePvE group Mobius for helping me hunt down the Imperial Capital Ship.
The ship sizes are taken from here